Substantially homogeneous copolymers of acrylic or methacrylic acid and maleic acid

ABSTRACT

Substantially homogeneous acrylate/maleate copolymers of number average molecular weight of 500 to 5000 are prepared by copolymerizing 35 to 65 mole percent acrylic or methacrylic acid with 65 to 35 mole percent maleic anhydride at 80° to 150° C. in the presence of a polymerization initiator and a chain-transfer solvent such that both the monomers and the formed polymer remain in solution and the mole ratio of the acrylic or methacrylic acid monomer to the maleic anhydride monomer in the solvent is maintained below about 0.2 throughout the polymerization. The copolymers in hydrolyzed form are employed at a level of from about 0.1 to 100 ppm for prevention of alkaline calcium and magnesium scale formation, such as during seawater evaporative desalination.

This is a division of application Ser. No. 176,668, filed on Mar. 18,1983, now U.S. Pat. No. 4,547,559.

BACKGROUND OF THE INVENTION

This invention relates to novel copolymers of maleic anhydride withacrylic or methacrylic acid, a process for their preparation and theiruse in hydrolyzed form as antiscalants.

Aqueous systems containing dissolved mineral salts are commonly employedin operations such as heating, cooling and evaporative distillationwhich involve heat transfer. The salts often become insoluble during theoperation and are deposited as scale on the heat transfer surfaces,resulting in reduced heat transfer and eventual failure of theequipment. In general, the mineral scale salts are derived from alkalineearth and other metal cations such as calcium, magnesium, iron and lead,and from anion such as bicarbonate, carbonate, hydroxide, sulfate andphosphate. Many factors influence scale formation and include, forexample, the pH of the water, the nature of the dissolved salts and thetemperature of the operation.

Numerous additives have been proposed as inhibitors of mineral scaling.Included are polymaleic acid and copolymers of maleic acid with such asacrylic and methacrylic acids. A variety of approaches to thepreparation of these acrylate/maleate copolymers has been reported, suchpreparations being disclosed, for example, in U.S. Pat. Nos. 3,635,915and 3,755,264, British patent specification No. 1,414,918 andNetherlands application No. 7506874. The disclosed preparations,however, result in a product which is not a true copolymer but rather amixture of essentially homopolymers. It is therefore an objective of thepresent invention to provide a process for the preparation ofsubstantially homogeneous acrylate/maleate copolymers which haveimproved antiscalant activity and reduced residual monomer content.

SUMMARY OF THE INVENTION

I have now found that by proper selection of the polymerization solventand by careful control of the monomer ratio in the solvent during thepolymerization, substantially homogeneous copolymers of acrylic ormethacrylic acid, or mixtures thereof, with maleic anhydride may bereadily prepared.

Accordingly, the present invention entails a process for preparing asubstantially homogeneous acrylate/maleate copolymer of number averagemolecular weight of from about 500 to 5000 which comprisescopolymerizing from about 35 to 65 mole percent acrylic or methacrylicacid monomer with from about 65 to 35 mole percent maleic anhydridemonomer over a period of from about 0.3 to 10 hours at a temperature offrom about 80° to 150° C. in the presence of from about 5 to 25 weightpercent polymerization initiator, based on the total weight of themonomers, and from about 45 to 90 weight percent of chain-transfersolvent, based on the total weight of the monomers, initiator andsolvent, the solvent and the mode of addition of the monomers to thesolvent being such that the monomers and copolymer remain in solution inthe solvent, and the mole ratio of the acrylic or methacrylic acidmonomer to the maleic anhydride monomer in the solvent is maintained atfrom about 0.005 to 0.2, throughout the polymerization period.

In preferred embodiments of the process, from about 40 to 50 molepercent methacrylic acid monomer is copolymerized with from about 60 to50 mole percent maleic anhydride monomer; the initiator is di-t-butylperoxide; the solvent is selected from the group consisting of ketones,esters and ethers having from 4 to 10 carbon atoms, and mixturesthereof; and the mole ratio of the acrylic or methacrylic acid monomerto the maleic anhydride monomer in the solvent is maintained at fromabout 0.01 to 0.1. Preferably, the maleic anhydride monomer, theinitiator and up to about 10 percent of the acrylic or methacrylic acidmonomer are dissolved in the solvent and the remainder of the acrylic ormethacrylic acid monomer is introduced over the polymerization period;the resulting copolymer solution is maintained at the polymerizationtemperature for from about 10 to 120 minutes following the monomeraddition; from about 40 to 100 weight percent water, based on the weightof the final polymerization solution, is added to the solution whilemaintaining the polymerization temperature to hydrolyze the copolymer;and the hydrolyzed copolymer is recovered as an aqueous solution byremoval of the solvent such as by distillation.

In an especially preferred embodiment of the process, from about 40 to45 mole percent methacrylic acid monomer is added over a period of fromabout 2 to 4 hours to a solution of from about 60 to 55 mole percentmaleic anhydride monomer, 10 to 20 weight percent di-t-butyl peroxideand 50 to 60 weight percent methyl isobutyl ketone at a temperature offrom about 110° to 120° C., and the solution is held at that temperaturefor from about 15 to 90 minutes following the addition.

The present invention also entails a substantially homogeneous copolymerconsisting essentially of from about 35 to 65 mole percent acrylic ormethacrylic acid units and from about 65 to 35 mole percent maleicanhydride units and having a number average molecular weight of fromabout 500 to 5000. Preferably the copolymer has from about 40 to 45 molepercent methacrylic acid units and from about 60 to 55 mole percentmaleic anhydride units, and is in the hydrolyzed form, or as an alkalimetal, ammonium or amine salt thereof, as an aqueous solution containingfrom about 1 to 60 weight percent of the hydrolyzed copolymer.

The present invention further entails a method for the prevention ofscale formation in water containing scale-forming impurities, such as inseawater for desalination, which comprises mixing with the water aneffective amount of the hydrolyzed copolymer, preferably at aconcentration of from about 0.5 to 10 ppm. The method may also compriseaddition to the seawater of an amount of such as sulfuric acid toneutralize preferably from about 55 to 85 percent of the bicarbonatealkalinity of the seawater.

BRIEF DESCRIPTION OF THE DRAWINGS

Novel features and advantages of the present invention in addition tothose mentioned above will become apparent from a reading of thefollowing detailed description in conjunction with the accompanyingdrawings wherein:

FIG. 1 is a typical HPLC curve for a substantially homogeneousmethacrylate/maleate copolymer prepared according to the presentinvention; and

FIG. 2 is a typical HPLC curve for a methacrylate/maleate copolymerproduct prepared by a prior art copolymerization process.

DETAILED DESCRIPTION OF THE INVENTION

The novel polymerization process of the present invention, as describedherein, provides for the first time essentially homogenous copolymers ofcontrolled molecular weight from the copolymerization of major amountsof acrylic or methacrylic acid with maleic anhydride. In addition, thesenovel copolymers exhibit improved antiscalant activity and lowerresidual monomer content over the acrylate/maleate copolymers of theprior art, resulting in more efficient utilization and a greater degreeof safety in their use, such as during the flash evaporation of seawaterin preparing potable water.

Homogeneous copolymers are defined by the art as compositions of narrowmolecular weight distribution. The distribution is readily determined bysuch analytical techniques as high pressure liquid chromatography (HPLC)and is essentially mononodal for homogeneous copolymers. Nonhomogeneouscopolymers, on the other hand, characteristically have binodaldistributions, particularly when the reactivities of the two monomersare very different as is the case with acrylic or methacrylic acid andmaleic anhydride. When determined by HPLC, the molecular weightdistribution of the substantially homogeneous copolymers of the presentinvention is essentially mononodal with the ratio of the weight averagemolecular weight (MW_(W)) to the number average molecular weight(MW_(N)) being less than 6.

In this process, from about 35 to 65 mole percent of either acrylic acidor methacrylic acid is copolymerized with from about 65 to 35 molepercent of maleic anhydride in the presence of specified amounts ofchain-transfer solvent and polymerization initiator and at specifiedtemperatures and polymerization times, the nature of the solvent and themethod of addition of the monomers being selected to ensure that thepolymerization mixture is essentially a solution at all times and thatthe ratio of the more reactive acrylic or methacrylic acid monomer tothe maleic anhydride monomer in the solvent is maintained at a low valueand within a specified range.

The chain-transfer solvent selected for the polymerization must meetcertain criteria. It must be such that both the monomers and theresulting copolymer have substantially complete solubility in thesolvent. It must be substantially unreactive to carboxylic acid andanhydride groups, thus excluding such solvents as alcohols and amines.It must be substantially unreactive to the monomer double bonds forthose reactions of anionic nature such as addition of amines or thiols.Finally, it must be capable of radical chain transfer from the initiatorand from the growing polymeric radical in order that the molecularweight of the formed polymer is kept low. A desirable, but notessential, characteristic of the solvent is a normal boiling point inthe range of from about 80° to 150° C. This is so that the temperatureis high enough without operating under pressure to cause a suitableradical production rate from initiator thermal decomposition; that lowrather than high molecular weight polymer is formed; and that thesolvent is readily removed from the copolymer product. In addition,water insolubility of the solvent is preferred to permit facile recoveryof the copolymer product by extraction into the aqueous phase.

Preferably, the solvent is selected from ketones, esters and ethershaving from 4 to 10 carbon atoms. Thus, ketones such as methyl isobutylketone, n-pentanone and n-hexanone, esters such as n-butyl acetate,diethyl phthalate and methyl benzoate, and ethers such as1,2-diethoxyethane, 1,4-dioxane and diisopropyl ether, or mixturesthereof, may be used. The especially preferred solvent is methylisobutyl ketone. The amount of chain transfer solvent employed is fromabout 45 to 90 weight percent of the total weight of the monomers,initiator and solvent.

To obtain a substantially homogeneous acrylate/maleate copolymer of thedesired composition, a low ratio of the more reactive acrylic ormethacrylic acid monomer to the maleic anhydride in the chain-transfersolvent must be maintained throughout the polymerization. This ratio maybe calculated by the monomer reactivity ratio method described by P. J.Flory in "Principles of Polymer Chemistry", Cornell University Press,1953, page 178. Employing this method together with available monomerreactivity ratios for copolymerizations of maleic anhydride and ofmethacrylic acid, it is calculated that the ratio of acrylic ormethacrylic acid monomer to maleic anhydride monomer during thepolymerization should be from about 0.005 to 0.2 for copolymerscontaining from about 35 to 65 mole percent maleic anhydride units withfrom about 65 to 35 mole percent acrylic or methacrylic acid units, andfrom about 0.01 to 0.1 for the preferred copolymers containing fromabout 40 to 60 mole percent maleic anhydride units with from about 60 to40 mole percent methacrylic acid units.

In carrying out the present invention, it will often be appropriate toconduct one or more initial pilot polymerization runs in which reactionconditions are carefully recorded and in which samples of the reactionmixture are periodically withdrawn, rapidly chilled and assayed foracrylic or methacrylic acid monomer and maleic anhydride monomercontent. In this fashion, it can be confirmed that the criticalparameters of the invention have been achieved with regard to the ratioof acrylic or methacrylic acid monomer to maleic anhydride monomer. Insubsequent runs, it will not be necessary to repeat the samplingprocedure, but instead it will be most convenient to merely observe thevarious reaction parameters which conform to the successful pilot run.In practice, it has been found that the homogeneous copolymer of thepresent invention is achieved when the solvent, catalyst, total maleicanhydride monomer and up to 10 percent of the acrylic or methacrylicacid monomer are charged to the reaction vessel, the remainder of theacrylic or methacrylic acid monomer being introduced continuously overthe course of the polymerization.

The polymerization initiator is preferably selected from dialkylperoxides such as di-t-butyl peroxide, dicumyl peroxide,2,5-dimethyl-2,5-bis(t-butylperoxy) hexane and2,5-dimethyl-2,5-bis(t-butylperoxy) hexyne-3 and from diacyl peroxidessuch as benzoyl peroxide, lauroyl peroxide, 2,4-dichlorobenzoyl peroxideand succinic acid peroxide. The initiator is utilized at a level of fromabout 5 to 25 percent by weight of the combined monomers and is normallyadded in its entirety at the start of the polymerization. The preferredinitiator is di-t-butyl peroxide at a level of about 10 to 20 weightpercent of the monomer charge.

The temperature and duration of the polymerization are also influentialin determining the nature of the resulting copolymer. The polymerizationis therefore limited to temperatures of from about 80° to 150° C. formonomer addition periods of from about 0.3 to 10 hours. Since thepresence of a minimum amount of monomer in the final product is desired,the final polymerization solution is generally maintained at thepolymerization temperature for a period, preferably from about 10 to 120minutes, following the completion of the monomer addition.

Preferred embodiments of the instant process include those in which fromabout 40 to 50 mole percent methacrylic acid is copolymerized with fromabout 60 to 50 mole percent maleic anhydride. In an especially preferredprocess, about 40 to 45 mole percent methacrylic acid is addedcontinuously to a refluxing solution of from about 60 to 55 mole percentmaleic anhydride and 10 to 20 weight percent di-t-butyl peroxide in fromabout 50 to 60 weight percent methyl isobutyl ketone at a temperature atfrom about 110° to 120° C. for from about 2 to 4 hours, and the finalpolymerization solution is held at the reflux temperature for anadditional 15 to 90 minutes following completion of the methacrylic acidaddition.

The essentially homogeneous copolymer consisting essentially of fromabout 35 to 65 mole percent acrylic or methacrylic acid units and fromabout 65 to 35 mole percent maleic anhydride units and having a numberaverage molecular weight of from about 500 to 5000 may be isolated fromthe final polymerization solution by techniques well known to the art.But since the copolymer is conventionally used in hydrolyzed form, theinstant process preferably entails hydrolyzing the copolymer by theaddition of water to the final polymerization solution, normally in theamount of from about 40 to 100 weight percent of the solution, while thesolution is still at the polymerization temperature. The resultingmixture may be held at or near the polymerization temperature, generallyfor a period of up to about 60 minutes, to assure complete hydrolysis.The solvent is then removed from the mixture, such as by phaseseparation or preferably distillation, depending upon the nature of thesolvent, and the hydrolyzed copolymer recovered as such or in the formof an aqueous solution.

The hydrolyzed copolymer aqueous solution may be used as such orneutralized to form a solution of the alkali metal, ammonium or aminesalt. Alkali metal bases suitable for the neutralization include sodiumhydroxide, potassium hydroxide and lithium hydroxide, while suitableammonium and amine bases include ammonia, ammonium hydroxide, mono-, di-and trialkyl amines having 1 to 5 carbon atoms in each alkyl group,pyridine, morpholine and lutidine. As an alternative to neutralizationof the hydrolyzed copolymer solution in forming the salt, the base,except in the case of mono- and dialkyl amines, may be added with thehydrolysis water.

The essentially homogeneous hydrolyzed acrylate/maleate copolymer of thepresent invention, while having general scale-inhibiting properties, isespecially useful in the prevention of alkaline calcium and magnesiumscale formation in such processes as the desalination of seawater. Insuch use, the hydrolyzed copolymer is added to the water being treatedin an effective amount, normally at a level from about 0.1 to 100 ppmand preferably from about 0.5 to 10 ppm, to inhibit scale formation. Insuch addition, the aqueous copolymer solution at about 1 to 60 weightpercent hydrolyzed copolymer or salt equivalent thereof may be metereddirectly into the stream being treated, but preferably is diluted withwater to a concentration of about 0.1 to 1 weight percent before beingadded. Evaluation of these essentially homogeneous, controlled lowmolecular weight copolymers has shown them to be superior to theacrylate/maleate copolymers of the prior art prepared by heterogeneouspolymerization procedures.

Other additives commonly used in combination with threshold scaleinhibitors are also effective with the novel inhibitors of the presentinvention. Especially useful are mineral acids, especially sulfuricacid, whereby part of the bicarbonate alkalinity present in such asseawater to be desalinated is neutralized; the extent of the bicarbonateneutralization is normally from about 30 to 85 percent, especially fromabout 55 to 85 percent. Foam control agents, corrosion inhibitors andoxygen scavengers, for example, may also be utilized with the novelscale control agents of the present invention.

The following examples are merely illustrative and are not to beconstrued as limiting the invention, the scope of which is defined bythe appended claims.

EXAMPLE 1

To a 4-neck, 500 ml round bottom flask equipped with two refluxcondensers, thermometer, nitrogen inlet, mechanical stirrer andconstant-addition syringe drive pump were charged 204 g methyl isobutylketone, 100 g (1.02 mole) maleic anhydride and 25 g di-t-butyl peroxide.The flask was purged with dry nitrogen while the resulting solution washeated under agitation to reflux (about 115° C.), the system beingmaintained thereafter under a static nitrogen pressure. The continuousaddition of 58 g (0.67 mole) of glacial methacrylic acid to the solutionwas started once the solution temperature reached 100° C., the acidaddition continuing over a period of 3.3 hours. The polymerizationsolution was refluxed for 1.5 hours following completion of themethacrylic acid addition. Then 215 g of water was added to therefluxing solution, and the resulting mixture was distilledatmospherically to remove the methyl isobutyl ketone solvent and aportion of the water from the flask and produce an aqueous solution ofhydrolyzed copolymer containing about 50 weight percent solids. Theyield of hydrolyzed copolymer was 113 percent by weight based on thecharged monomers, the greater than 100 percent yield resulting from theconversion of the anhydride to acid groups and the incorporation ofsolvent end groups into the polymer.

The chemical homogeneity and molecular weight distribution of thehydrolyzed copolymer were determined by high pressure liquidchromatography (HPLC) on a series of three 27.5 cm 60 A° Porasil gelpermeation columns using an acetate-phosphate buffer eluant adjusted topH 7.4 with 1 N aqueous sodium hydroxide and refractive index detection,the Porasil columns being calibrated with 1,2,3,4-butane-tetracarboxylicacid and Goodrite K 732 and K 752 polyacrylates of known molecularweight. The analysis, as shown graphically in FIG. 1, indicated thecopolymer to be homogeneous with a weight average molecular weight of2970 and a number average molecular weight of 960.

EXAMPLE 2

The preparation of Example 1 was repeated with the exception that 48.2 g(0.67 mole) of acrylic acid was substituted for the methacrylic acid. Ahomogeneous hydrolyzed acrylate/maleate copolymer of weight averagemolecular weight of 2730 and number average molecular weight of 890 wasobtained in 119 weight percent yield.

EXAMPLE 3

The preparation of Example 1 was repeated with the exception that 204 gof 1,2-diethoxyethane (boiling point 121° C.) was substituted for themethyl isobutyl ketone solvent. Analysis of the aqueous copolymersolution indicated a 129 weight percent yield of homogeneous hydrolyzedmethacrylate/maleate copolymer of weight average molecular weight of1690 and number average molecular weight of 510.

EXAMPLE 4

To a 4-neck, 500 ml round bottom flask equipped as in Example 1 wereadded 50 g (0.51 mole) maleic anhydride, 8.0 g di-t-butyl peroxide and150 g n-butyl acetate. The reaction system was purged with nitrogen andheated to reflux (about 120° C.). Once at reflux, 44 g (0.51 mole)methacrylic acid was added continuously to the solution over a 1-hourperiod. Reflux was maintained for an additional 7 hours. Then 220 g ofwater was added to the polymerization solution and the resulting mixturewas held at about 90° C. for 1 hour. Agitation was stopped, the layerswere allowed to separate, and the upper organic layer was decanted. Thelower aqueous layer was reheated to and held at reflux (100° C.) for 6hours, then atmospherically distilled to removed about 100 g ofdistillate and produce a 43 weight percent aqueous solution ofhomogeneous hydrolyzed methacrylate/maleate copolymer in 92 weightpercent yield.

EXAMPLE 5

In a 3-neck, 300 ml round bottom flask equipped with reflux condenser,nitrogen inlet, mechanical stirrer and pressure-equalizing droppingfunnel were combined 58.8 g (0.60 mole) maleic anhydride, 14.6 gdi-t-butyl peroxide, 115 g methyl isobutyl ketone and 3.4 g (0.040 mole)glacial methacrylic acid. The system was purged with nitrogen and thenheated to reflux under a static positive nitrogen pressure. A smallamount of solids momentarily precipitated and then redissolved at about100° C. When the reaction mixture reached reflux (about 115° C.), theremaining 31.0 g (0.36 mole) methacrylic acid in 60 g of methyl isobutylketone was added continuously over a 3-hour period. The reactionsolution was refluxed for 15 minutes following the addition, 150 g ofwater was then added, and the ketone-water mixture refluxed for anadditional hour. The isolation procedure of Example 4 was followed toproduce a 57 weight percent aqueous solution of homogeneous hydrolyzedmethacrylate/maleate copolymer of weight average molecular weight of3390 and number average molecular weight of 610 in 98 weight percentyield.

EXAMPLE 6

Methacrylate/maleate copolymers were prepared by the procedures ofExamples 1 and 4 varying the initiator concentration and the methacrylicacid addition times as indicated in Table 1. Homogeneous hydrolyzedcopolymers having molecular weights indicated in the table wereobtained.

                  TABLE 1                                                         ______________________________________                                             Proce-              Addition                                                                              Product.sup.(2)                              Ex.  dure     Init wt %.sup.(1)                                                                        Time, hr                                                                              MW.sub.W                                                                             MW.sub.N                              ______________________________________                                        6A   1         5         5.0     5280   2080                                  6B   1         7         3.25    3900   1540 -6C 1 11 3.25 3600 1300          6D   1        14         3.25    3000    980                                  6E   4        19         3.0     3370   1350                                  6F   4         8         1.0     --     --                                    6G   4        14         6.0     3240   1110                                  ______________________________________                                         .sup.(1) initiator, weight percent of combined monomers                       .sup.(2) product copolymer weight average molecular weight (MW.sub.W) and     number average molecular weight (MW.sub.N)                               

EXAMPLE 7

The preparation of Example 1 is repeated with the exception that 54 g(0.55 mole) maleic anhydride is copolymerized with 39 g (0.45 mole)methacrylic acid in the presence of 23 g benzoyl peroxide initiator and95 g 2-butanone (boiling point 80° C.) as solvent, the methacrylic acidis added continuously over a 5-hour period, the refluxing is continuedfor 10 minutes following the methacrylic acid addition and the copolymeris hydrolyzed by the addition of 210 g water at reflux temperature. Asubstantially homogeneous methacrylate/maleate copolymer similar to theproduct of Example 1 is obtained.

Repetition of the preparation using 13 g di-t-butyl peroxide rather thanbenzoyl peroxide as initiator, 95 g 2-pentanone (boiling point 102° C.)rather than 2-butanone as solvent and an addition period of 10 hoursrather than 5 hours gives similar results.

EXAMPLE 8

The preparation of Example 4 is repeated with the exception that 39 g(0.40) mole maleic anhydride is copolymerized with 52 g (0.60 mole)methacrylic acid in the presence of 14 g di-t-butyl peroxide initiatorand 945 g diethyl phthalate in a 2000 ml flask, the methacrylic acid isadded over a 20-minute period at a polymerization temperature of 150°C., the reaction solution is held at the polymerization temperature for10 minutes following the methacrylic acid addition, and the copolymer ishydrolyzed by the addition of 420 g water added at a temperature of 120°C. A substantially homogeneous hydrolyzed methacrylate/maleate copolymeris obtained.

COMPARATIVE EXAMPLE 1

In a 3-neck 500 ml round bottom flask equipped with reflux condenser,nitrogen inlet, thermometer and mechanical stirrer were added 58.8 g(0.60 mole) maleic anhydride, 34.4 g (0.40 mole) methacrylic acid, 120 gmethyl isobutyl ketone and 14.6 g di-t-butyl peroxide. The flask waspurged with dry nitrogen for five minutes, then maintained under apositive nitrogen pressure while the reaction mixture was heated toreflux. At about 98° C. the reaction became exothermic and a relativelylarge amount of solids precipitated. The mixture was refluxed (115° C.)for four hours, and the hydrolyzed aqueous copolymer was isolated by theprocedure of Example 1. An aqueous hydrolyzed copolymer product showinga distribution with high and low molecular weight components asdetermined by HPLC was obtained in a weight yield of 119 percent.

COMPARATIVE EXAMPLE 2

The preparation of Example 4 was repeated substituting xylene for then-butyl acetate solvent. A rubbery mass precipitated during themethacrylic acid addition, so seriously impeding the stirring that thestirring was discontinued. A nonhomogeneous product was isolated in 132weight percent yield.

COMPARATIVE EXAMPLE 3

Methacrylate/maleate and acrylate/maleate (27/73 mole percent)copolymers were prepared in toluene and xylene, respectively, followingExamples 1 and 2 of British patent specification No. 1,414,918. Thepreparations, as shown graphically in FIG. 2 for themethacrylate/maleate copolymer, resulted in nonhomogeneous copolymerproducts in weight yields of 131 and 155 weight percent, respectively.

COMPARATIVE EXAMPLE 4

Methacrylate/maleate (43/57 mole percent) copolymer was preparedfollowing Example 1 of Netherlands patent application No. 7506874. Thepolymerization in toluene with benzoyl peroxide initiator was mildlyexothermic and heterogeneous. The resulting copolymer wasnonhomogeneous.

COMPARATIVE EXAMPLE 5

Methacrylate/maleate (12/88 mole percent) copolymer was preparedfollowing Example 1 of U.S. Pat. No. 3,755,264. The polymerizationconducted in toluene resulted in a precipitated polymer gummy in natureand a nonhomogeneous hydrolyzed copolymer isolated in 125 weight percentyield.

COMPARATIVE EXAMPLE 6

Acrylate/maleate (80/20 mole percent) copolymer was prepared in waterfollowing the procedure of Example 1 of U.S. Pat. No. 3,635,915. Theproduct, essentially a high molecular weight polyacrylate mixed withmore than 50 percent of the initially charged maleic anhydride as maleicacid, was obtained in 100 percent weight yield.

EXAMPLE 9

The methacrylate/maleate and acrylate/maleate copolymers of thepreceding examples were tested in a laboratory single-stage flashevaporator to determine their scale control performance. The operationof the laboratory evaporator and the method of testing was as describedby Auerbach and Carruthers in Desalination, 31, 279 (1979). BothAtlantic Ocean seawater and a synthetic seawater were used in thetesting, the latter being the "Standard Synthetic Seawater Composition"of the Office of Saline Water containing an augmented bicarbonatecontent of 0.25 g/kg.

Results of the testing are summarized in Table 2. The copolymerantiscalant dosage is reported in parts per million (ppm) of activesolids. The percent deposited scale is calculated from the equation:##EQU1## Copolymers which provide lower "% deposited scale" at a givendosage afford superior performance.

                  TABLE 2                                                         ______________________________________                                        Copolymer                                                                              Dosage,                                                              Example  ppm active                                                                              Brine       % Deposited Scale                              ______________________________________                                        1        3         synthetic   12.4                                           4        3         synthetic   15.8                                           Comp. Ex 1                                                                             3         synthetic   22.0                                           Comp. Ex 2                                                                             3         synthetic   18.1                                           Comp. Ex 3                                                                             3         synthetic   26.5                                           6A       4         Atlantic Ocean                                                                            17.6                                           6B       4         Atlantic Ocean                                                                            16.0                                           6C       4         Atlantic Ocean                                                                             7.7                                           6D       4         Atlantic Ocean                                                                             5.3                                           ______________________________________                                    

These test results clearly show the scale-inhibiting superiority of theessentially homogenous, controlled low molecular weight acrylate/maleatecopolymers of the present invention over the nonhomogeneous copolymersof the prior art prepared under heterogeneous conditions.

I claim:
 1. A substantially homogeneous copolymer consisting essentiallyof from about 35 to 65 mole percent acrylic or methacrylic acid repeatunits and from about 65 to 35 mole percent maleic acid repeat units andhaving a number average molecular weight of from about 500 to 5000, oran alkali metal, ammonium or amine salt thereof.
 2. An aqueous solutioncontaining the copolymer of claim 1 in the amount of from about 1 to 60percent by weight.